(First Background Art)
Reflective liquid crystal display devices reflect ambient light from a reflector that is disposed inside the liquid crystal display device to provide a display. No backlight as a light source is required accordingly in contrast to conventional transmissive liquid crystal display devices. Consequently, an increased number of them are mounted on cell phones and portable information terminals, as a display device that can reduce consumption power as compared with transmissive liquid crystal display devices.
Reflective liquid crystal display devices comprises a liquid crystal display mode of, for example, a TN (twisted nematic) type, an STN (suoer twisted nematic) type, a GH (guest-host) type, or a PDLC type (polymer dispersed liquid crystal), a substrate having TFTs (thin flat transistors) or TFDs (thin flat diodes) that are cells for writing a video signal or a simple matrix substrate, and a reflector that is disposed inside or outside the liquid crystal display panel to reflect ambient light.
Reflective liquid crystal display devices have several requirements to provide good display qualities, of which white display is most important. Of the above-mentioned liquid crystal display modes, it is required from the display performance viewpoints that bright and white display can be provided when a liquid crystal material layer is under the state of transmitting the ambient light. In order to achieve such display performances, control of reflection characteristics of a reflector is an important factor.
Mirror-like reflectors largely depend on viewing angles because the display is bright only in a direction, that is, a regular reflection direction, along which incident lights reflect as reflected lights. Thus, diffusivity reflectors currently prevail that allow bright display in a certain range of viewing angles. In order to impart diffusivity to the reflector, a concave/convex shape is usually formed on a surface of the reflector.
As to the concave/convex shape on the surface of the reflector, the following facts are already known.
(1) The pitch of a convex shape is in the range of from 1 μm to 100 μm, the height of the irregularities is in the range of from 0.1 μm to 10 μm, and the tilt angle of the irregularities is from 0 degrees to 30 degrees, with respect to a horizontal surface of a substrate, in which the distance between peaks of the irregularities is not constant (Japanese Patent Publication No. 61-6390).
(2) In a reflector having a irregular concave/convex structure, a value obtained by standardizing a half-value width of a distance distribution graph between adjacent convex portions or concave portions with an average value of the distances between the adjacent convex portions or concave portions is in the range of from 0.3 to 0.9 (Japanese Patent Laid-open No. 8-184846).
(3) In a reflector having a irregular concave/convex structure, an average distance between adjacent convex portions or concave portions is in the range of from 1 μm to 80 μm (Japanese Patent Laid-open No. 8-184846). Thus, a random arrangement of concave/convex structure is an essential requirement of the conventional art.
However, reflectors are considered to be optimum when they control the diffusion characteristics such that the diffusivity is limited within a certain range of viewing angle and diffused light is concentrated to this specified range to provide uniform brightness in this range (N. Sugiura and T. Uchida, AM-LCD95 digest, 153 (1995); T. Uchida, AM-LCD95 digest, 23 (1995)).
When concave/convex structures are arranged irregularly during the control of the diffusion characteristics to become optimum, a flat portion between the concave/convex structures becomes relatively large. The amount of reflected lights in the regular reflection becomes relatively large with respect to the incident lights, accordingly.
On the other hand, when the concave/convex structures have regularity or recurrency to reduce the percentage of an area occupied by the flat portion, these configurations serve as a diffraction grating. The diffraction grating emits light having high intensity at a certain wavelength in a certain direction. Thus, rainbow-like coloring is sensed and white diffusivity is blocked due to the wavelength dependence of the diffraction grating.
(Second Background Art)
With rapid growth of mobile terminals, reflective liquid crystal panels are getting a lot more attention. The reflective liquid crystal panel reflects ambient light to provide a display. Sufficient display characteristics are achieved in environments such as outdoors where strong ambient light is available. However, visibility is significantly lower in a dark indoors or during night time.
Accordingly, in a reflective liquid crystal panel, it is desirable to improve reflectance in the direction of a viewer by using a diffusive reflector that reflects and concentrates lights received from peripheries toward the viewer. As means to achieve this, a technique to form a plurality of concave/convex structures on a pixel electrode is disclosed (Japanese Patent Laid-open No. 5-281533). In this event, concave/convex structures having a plurality of layers are formed through repeated processes of coating and exposure of a photosensitive polymer film, a development treatment, and a subsequent thermal annealing treatment.
The reflectance of a panel in the direction of a viewer depends on a concave/convex shape of a reflective layer. With ideal reflection characteristics, it is necessary to keep a high luminance in the range of from perpendicular to a polar angle of 30°.
In order to provide ideal reflectance characteristics, a sloped surface of a convex portion of the concave/convex shape is required to be precipitous as in a generally triangle shape. However, no technique is known conventionally to easily form a sloped surface of the concave/convex shape into a generally triangle shape. In addition, the formation of the concave/convex shapes conventionally involves repeated processes of coating and exposure of a photosensitive polymer film, and development and thermal annealing treatments. The processes are complicated and there is a challenge associated with productivity.
(Third Background Art)
In recent years, as AV equipment and information equipment has been reduced in size and thickness, demands on liquid crystal display panel for such equipment has been increased. For the information equipment, liquid crystal display panels that are available for more portable notebook personal computers are required, with the advent of the multimedia society. In the field of portable information terminals, a thinner and lighter liquid crystal display panel that consumes less power has been desired. In particular, reflective liquid crystal display panels require no light source such as a backlight unit because they reflect ambient light to show images. Accordingly, the reflective liquid crystal display panels have a chance of being reduced in side, weight, and consumption power, as compared with conventional transmissive liquid crystal display panels. As display modes for the reflective liquid crystal display panels, the TN (twisted nematic) method, the STN (super twisted nematic) method, a guest-host method with dichroic pigments are used more frequently.
In the reflective liquid crystal display panels, the intensity of light that reflects and diffuses incident lights in a perpendicular regular angle of visibility of a display screen should be increased in order to provide brighter and better display. Furthermore, it is preferable that the incident lights reflect and diffuse, in the direction of the regular angle of visibility, ambient light that is directed from a predetermined direction at a certain angle as well as reflect and diffuse, in the direction of the regular angle of visibility, the ambient light that is directed from various directions at random angles. Accordingly, it becomes necessary to prepare a reflector having optimum reflection characteristics that allows efficient use of the ambient light directed from any direction as display light. The term “optimum reflection characteristics” as used herein means that the reflector has characteristics of reflecting incident lights over a wide range with high reflectance.
Using conventional reflectors such as those having a mirror-faced metal film deposited on a substrate, the incident lights reflect only in the regular reflection direction. The reflectance is low in the directions other than the direction of the regular reflection. Consequently, the display screen is significantly dark in a viewing direction of a viewer such as a direction of the regular angle of visibility, which brings about drop in visual quality.
Against such problems, a picture element electrode having reflection characteristics that reduce reflection of incident lights to a regular reflection region is disclosed in, for example, Japanese Patent Laid-open No. 6-27481. According to this publication, as shown in FIG. 42, a reflector has a configuration in which a polymer resin film 2103 is formed on a substrate 2101 where a plurality of convex portions 2112a and 2112b are formed, and picture element electrodes 2104 are formed on the polymer resin film 2103. In addition, the surface of the picture element electrodes 2104 has a continuous wave shape.
The above-mentioned reflector may be formed by the following method. First, as shown in FIG. 42(a), a resist film 2112 made of a photosensitive resin is coated by spin coating on the substrate 2101. Subsequently, it is pre-baked at a predetermined processing temperature. Then, as shown in FIG. 42(b), a photomask 2105 is used and is placed above the resist film 2112 for exposure. Subsequently, a developing solution is used to make development. As shown in FIG. 42(c), the convex portions 2112a and 2112b which are different in height are formed on the substrate 2101. Then, as shown in FIG. 42(d), the convex portions 2112a and 2112b are heated for 1 hour at a predetermined temperature for the heat treatment. The above-mentioned operation produces convex portions 2112a and 2112b that corresponds to the convex portions 2112a and 2112b whose corners are chamfered. Next, as shown in FIG. 42(e), a polymer resin is spin coated on the substrate 2101 after the heat treatment to form the polymer resin film 2103. Finally, the picture element electrodes 2104 are formed by sputtering on the polymer resin film 2103 (FIG. 42(f)).
By the above-mentioned method, the tilt angle (the angle formed between tiny faces on the surface of a picture element electrode having irregularities and the surface of the substrate) of the convex portions 2112a and 2112b is controlled to improve the reflection characteristics.
However, in the above-mentioned conventional manufacturing method, the control of the tilt angle works in theory but a desired reflector cannot be manufactured due to too small manufacturing process margins. More specifically, in practical manufacturing process, a reflector having a desired tilt angle cannot be manufactured due to, for example, an error of a heating temperature or an error of heating time. Therefore, the reflectors obtained using the conventional manufacturing method are insufficient in brightness in the direction of the angle of visibility. Good PAPER WHITE APPEARANCE is not obtained over a wide range. Thus, the size of the manufacturing process margin is an important issue for the manufacturing process in practice.
In the conventional manufacturing method, reflow caused by heat is used to control the shape and apply a resist of the second layer. This results in increased tact time and increased costs.